Im sure there are people out there that did not know...however... prior to the dmg of MS hitting your target (unless CC'd) MSing the tank was the 'good' choice option. That said, I personally still use it on tanks when i feel less urgy to actually target something, or whoever is in range (mages on Alysrazor)

The amount of times I have told the spriest in LFG pugs and raid pugs to target the tank when aoeing because he stupidly targets the skull as his mind sear target is just ridiculous, and then they tell me "it's not possible, mind sear doesn't work like that".

While I was leveling my prot warr the other months, almost none of the priests I met were aware of this. And I'd tell them "use MS on me, it's more efficient". And they were like :O . I was happy I could contribute to their knowledge :P

While targeting the tank is a better option it really doesn't matter if you target an enemy mob instead since Mind Sear do deal damage to the target aswell (which it didn't before) plus the other ones around it. But if you've followed news posts and watched video kills of bosses it's kind of hard to miss the information that you can use Mind Sear to target a friendly mob. i.e. Paragon, on the ragnaros hc fight, used their shadow priests to channel Mind Sear on the pets near the seeds on the seed phases, to deal damage right when they pop up and I'm sure many other guilds have done the same.

But yea, thank you Blizzard for giving the option to target friendly mobs/players/npcs with mind sear. ^^

Why

I always thought the reason to target MS on the tank was mainly because otherwise if the mob you're focused on dies dies you have to re-target and recast... Which if done 3/4 times on a big pull is a significant dps loss and waste of mana